A helix resonator, known as a helix, is a quarter-wave transmission line resonator (electrical length about a quarter of a wavelength). The resonator is formed by a round or profiled wire wound into a cylindrical coil. The helix resonator is placed in a casing made of a highly conductive material and forming the ground plane, and one end of the helix is left open. The specific impedance of the helix resonator is determined by the ratio of the diameter of the coil to the inner dimension of the surrounding casing, the distance between adjacent turns in the coil, i.e., pitch, and the properties of the plastic or printed board material supporting the resonator.
The helix resonator is considerably smaller than so called coaxial resonators having similar properties, which is a factor of great importance in high-frequency radio devices By cascading several such resonators, a small high-frequency filter with good properties is obtained for use especially in portable and cellular radiotelephone devices. The rapid decrease in the size of such radio devices has created a demand for considerably smaller filters Decreased filter sizes, in turn, require greater accuracy in the production and assembly of high-frequency components.
In filter structures, the helix resonator has to be connected to a generator exhibiting a predetermined specific impedance or to a similar signal producing device and to other helix resonators in the filter circuit. Thereby the coupling between the generator and the resonator, for instance, has to be such that their impedances match in order to prevent mismatch losses in signal transmission. A suitable impedance level has to be found in the helix resonator, that is, a physical point of connection at which the impedance level of the resonator equals that of the device connected thereto. This is because the impedance level of the helix resonator is directly proportional to its electrical length, whereby a lower or higher helix resonator impedance level can be selected by moving the point of connection along the resonator. This kind of matching may be called "tapping" because the point of connection forms a tap point from the helix resonator. The tap or connection point in the helix resonator can be determined by experimentation or by calculation using a calculated or measured specific impedance of the helix resonator. In many cases, the tap point in the helix resonator is made in its first turn. Traditionally, tapping has been made by soldering or welding one end of a separate coil to the wire forming the helix resonator at the tap point. With decreasing filter sizes, the reproduction fidelity has been found to be inadequate for series production. Inadequate accuracy in tapping results in a need for adjusting the tapping when tuning the filters, which increases tuning time and costs.